How do you pass the parent entity to a form in Symfony?

Suppose I have two entities: a post and a comment. Each post can have many comments. Now, suppose I have a comment form. It is supposed to take user input and store it in the database.

Simple stuff. At least, it should be, but I can't get it to work.

How do I refer to the post (parent) when creating the comment (child)? I tried manually passing the post_id to the comment form as a hidden field, but received an error complaining about how the post ID is a string.

Expected argument of type "App\Entity\Post or null", "string" given.

---

Here is my code so far. Can someone nudge me into the right direction?

CommentType.php

public function buildForm(FormBuilderInterface $builder, array $options)
{
    $post_id = $options['post_id'];

    $builder->add('content', TextareaType::class, [
        'constraints' => [
            new Assert\NotBlank(['message' => 'Your comment cannot be blank.']),
            new Assert\Length([
                'min'        => 10,
                'minMessage' => 'Your comment must be at least  characters long.',
            ]),
        ],
    ])->add('post', HiddenType::class, ['data' => $post_id]);
}

public function configureOptions(OptionsResolver $resolver)
{
    $resolver->setDefaults([
        'data_class' => Comment::class,
        'post_id' => NULL,
    ]);
}

---

PostController.php (this is where the comment form appears)

// Generate the comment form.
$comment = new Comment();
$form = $this->createForm(CommentType::class, $comment, [
    'action' => $this->generateUrl('new_comment'),
    'post_id'   => $post_id,
]);

---

CommentController.php

/**
 * @param Request $request
 * @Route("/comment/new", name="new_comment")
 * @return
 */
public function new(Request $request, UserInterface $user)
{
    // 1) Build the form
    $comment = new Comment();
    $form = $this->createForm(CommentType::class, $comment);

    // 2) Handle the submit (will only happen on POST)
    $form->handleRequest($request);
    if ($form->isSubmitted() && $form->isValid())
    {
        // 3) Save the comment!
        $entityManager = $this->getDoctrine()->getManager();
        $entityManager->persist($comment);
        $entityManager->flush();
    }

    return $this->redirectToRoute('homepage');
}

---

Thank you very much for your help!

from How do you pass the parent entity to a form in Symfony?